(a) Field of the Invention
The present invention relates to a method for forming a dielectric film and, more particularly, to a method for forming a dielectric film having a relatively large thickness and a relatively large area. The present invention also relates to a system for forming a dielectric film on a substrate.
(b) Description of the Related Art
A plasma display panel (PDP) is known as a flat display panel especially suited to achieving a large screen, and generally used in a personal computer, a work station, or a wall TV.
A PDP has a front panel and a rear panel sandwiching therebetween a plurality of phosphors, a plurality of sustaining electrodes and a thick dielectric film, wherein the phosphors are formed on the rear panel and the sustaining electrodes and the thick dielectric film are consecutively formed on the front panel. In order to form the thick dielectric film on the front panel, paste including powder of a dielectric material is applied to the front panel by using a screen printing, die-coat or green sheet technique, followed by drying, heating of the same up to the softening temperature and the reaction temperature of the dielectric powder, and then baking the dielectric powder. In general, the dielectric film includes a plurality of layers consecutively formed, if the dielectric film has a large thickness.
The baking step is generally conducted at an atmospheric pressure, wherein the dielectric film is maintained at the baking temperature for a relatively long time if the dielectric film having a thickness around 30 micrometers or above is baked at a time. The baking temperature allows the dielectric film to have a high fluidity and thereby discharge bubbles from the dielectric film, whereby the dielectric film has a high transparency after the baking.
Patent Publication JP-A-8-17337 describes a method for heating and baking dielectric powder at a reduced ambient pressure of 0.27 to 0.40 Pa, for example, wherein the dielectric powder is allowed to effectively discharge cracked gas components during the heating.
In the described technique, if the dielectric powder is subjected to the baking at a lower ambient temperature and thus at a lower fluidity thereof, the degassing effect is relatively limited. To raise the degassing effect, it may be considered to conduct the baking for a long time, or a plurality of times for the single dielectric layer. However, this involves a lower throughput and higher costs for obtaining the dielectric film.
In addition, the lower fluidity may leave bubbles (or air gaps) having large dimensions in the resultant dielectric film due to the reduced pressure which generally expands the bubbles in the molten dielectric material. This involves reduction of breakdown voltage of the resultant dielectric film, especially in the case of low-melting-point glass, which is generally baked at around the softening temperature thereof. Further, if reduction of the transparency is associated with generation of the bubbles, the resultant PDP has a poor brightness in the discharge thereof.